Re: [REGRESSION] (>= v4.12) IO w/dmcrypt causing audio underruns

From: Enric Balletbo Serra
Date: Mon Dec 18 2017 - 04:25:41 EST

Hi Vito,

2017-12-01 22:33 GMT+01:00 <vcaputo@xxxxxxxxxxx>:
> On Wed, Nov 29, 2017 at 10:39:19AM -0800, vcaputo@xxxxxxxxxxx wrote:
>> Hello,
>> Recently I noticed substantial audio dropouts when listening to MP3s in
>> `cmus` while doing big and churny `git checkout` commands in my linux git
>> tree.
>> It's not something I've done much of over the last couple months so I
>> hadn't noticed until yesterday, but didn't remember this being a problem in
>> recent history.
>> As there's quite an accumulation of similarly configured and built kernels
>> in my grub menu, it was trivial to determine approximately when this began:
>> 4.11.0: no dropouts
>> 4.12.0-rc7: dropouts
>> 4.14.0-rc6: dropouts (seem more substantial as well, didn't investigate)
>> Watching top while this is going on in the various kernel versions, it's
>> apparent that the kworker behavior changed. Both the priority and quantity
>> of running kworker threads is elevated in kernels experiencing dropouts.
>> Searching through the commit history for v4.11..v4.12 uncovered:
>> commit a1b89132dc4f61071bdeaab92ea958e0953380a1
>> Author: Tim Murray <timmurray@xxxxxxxxxx>
>> Date: Fri Apr 21 11:11:36 2017 +0200
>> dm crypt: use WQ_HIGHPRI for the IO and crypt workqueues
>> Running dm-crypt with workqueues at the standard priority results in IO
>> competing for CPU time with standard user apps, which can lead to
>> pipeline bubbles and seriously degraded performance. Move to using
>> WQ_HIGHPRI workqueues to protect against that.
>> Signed-off-by: Tim Murray <timmurray@xxxxxxxxxx>
>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
>> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>> ---
>> Reverting a1b8913 from 4.14.0-rc6, my current kernel, eliminates the
>> problem completely.
>> Looking at the diff in that commit, it looks like the commit message isn't
>> even accurate; not only is the priority of the dmcrypt workqueues being
>> changed - they're also being made "CPU intensive" workqueues as well.
>> This combination appears to result in both elevated scheduling priority and
>> greater quantity of participant worker threads effectively starving any
>> normal priority user task under periods of heavy IO on dmcrypt volumes.
>> I don't know what the right solution is here. It seems to me we're lacking
>> the appropriate mechanism for charging CPU resources consumed on behalf of
>> user processes in kworker threads to the work-causing process.
>> What effectively happens is my normal `git` user process is able to
>> greatly amplify what share of CPU it takes from the system by generating IO
>> on what happens to be a high-priority CPU-intensive storage volume.
>> It looks potentially complicated to fix properly, but I suspect at its core
>> this may be a fairly longstanding shortcoming of the page cache and its
>> asynchronous design. Something that has been exacerbated substantially by
>> the introduction of CPU-intensive storage subsystems like dmcrypt.
>> If we imagine the whole stack simplified, where all the IO was being done
>> synchronously in-band, and the dmcrypt kernel code simply ran in the
>> IO-causing process context, it would be getting charged to the calling
>> process and scheduled accordingly. The resource accounting and scheduling
>> problems all emerge with the page cache, buffered IO, and async background
>> writeback in a pool of unrelated worker threads, etc. That's how it
>> appears to me anyways...
>> The system used is a X61s Thinkpad 1.8Ghz with 840 EVO SSD, lvm on dmcrypt.
>> The kernel .config is attached in case it's of interest.
>> Thanks,
>> Vito Caputo
> Ping...
> Could somebody please at least ACK receiving this so I'm not left wondering
> if my mails to lkml are somehow winding up flagged as spam, thanks!

Sorry I did not notice your email before you ping me directly. It's
interesting that issue, though we didn't notice this problem. It's a
bit far since I tested this patch but I'll setup the environment again
and do more tests to understand better what is happening.